Composite video signals, such as those used in typical television receivers, include video information which is intended for display on a device sucb as a cathode ray tube. The video information is interspersed between synchronizing pulses to synchronize scanning of the cathode ray tube in the receiver to provide a coherent display. A signal in accordance with current FCC standards has 525 horizontal scanning lines per vertical frame. The vertical frame is divided into two vertical fields, known as the odd and even fields, with each field having 262.5 horizontal lines. If this relationship is maintained exactly, the signal is called interlaced, standard or synchronous.
A typical prior art approach to synchronizing the scanning of a cathode ray tube with the synchronizing pulses of the composite video signal is to provide two oscillators synchronized respectively with the horizontal and vertical synchronizing pulses. While such circuitry has been used for many years with generally satisfactory results, known prior art vertical oscillators are susceptible to noise and have poor stability. Accordingly, with prior art oscillator circuitry, it is necessary to provide a user adjustable hold control to compensate for oscillator drift. Such compensation, however, is generally insufficient to provide proper interlace of the scanning signals under many normal operating conditions. Furthermore, noise signals often cause false triggering of prior art vertical oscillators which deleteriously affects scanning. Also, horizontal pickup by the vertical circuitry may disturb proper interlace.
A technique commonly used to eliminate the vertical oscillator is to provide a countdown circuit which counts pulses provided at, for example, twice the horizontal line rate. Every 525 counts of the counter a vertical output pulse is provided. Systems using a counter are generally called vertical countdown systems since they count down pulses synchronized with the horizontal synchronizing pulses. Of course, a provision must be made for initially synchronizing the counter with the vertical synchronizing pulses of the composite video signal. Also, a provision must be made for periodically testing to ensure that the counter remains synchronized with the vertical synchronizing pulses. Once the counter is properly synchronized, the vertical synchronizing pulses and attendant noise can be essentially locked-out of the system.
In typical prior art vertical countdown systems, the technique for testing to determine whether the counter is properly synchronized is to compare pulses derived from the counter with the vertical synchronizing pulses. If the pulses are out-of-phase, the counter is reset. Since one or more vertical synchronizing pulses may be missing, replaced or obscured by noise, or accompanied by noise, some prior art systems require a predetermined number of unsuccessful comparisons before the counter is resynchronized. In such systems, however, signals which do not maintain the 525:1 relationship cause the counter to be resynchronized on a more or less regular basis thereby causing vertical jitter of the display. Such jitter is highly objectionable and can make the display unviewable. Such non-interlaced, non-standard, or asynchronous signals may be provided, for example, in CATV systems, video games, video players, and the like. Since it is highly desirable to provide television receivers with a capability of accomodating non-standard signals, attempts have been made to detect such signals. When non-standard signals are detected, a driven mode of operation is provided so that the vertical output pulses are derived from the vertical synchronizing pulses rather than from the counter.
One prior art system for detecting non-standard signals detects the presence or absence of equalizing pulses or serrations of the vertical synchronizing pulses. It has been found, however, that this technique may be susceptible to noise which can erroneously cause the system to go into the countdown mode. Furthermore, this technique denies the benefit of a countdown mode of operation for signals with a 525:1 relationship which are not accompanied by equalizing pulses or a serrated vertical synchronizing pulse.
Another prior art technique permits the system to remain in the countdown mode until a predetermined number of successive pulses from the counter are non-coincident with the vertical synchronizing pulses. This system suffers from the disadvantages of being unable to correct the counter without reverting to the driven mode so that a non-standard signal very close to a 525:1 relationship will either cause the system to stay in a driven mode or will cause the system to drift between the driven and countdown modes thereby either denying the system the benefit of a countdown mode or causing vertical jitter. Phase errors can also accumulate causing a periodic resynchronization of the counter and attendant vertical jump or jitter of the display. Furthermore, in instances where the received signal is weak and noisy, such prior art systems tend to stay in the driven mode because the vertical synchronizing pulses are not sufficiently regular to permit the system to go into the countdown mode. Thus, the benefits of the countdown mode are denied for weak and noisy signals where the most benefit is derived from a countdown system.